1. Field of the Invention
The present invention relates to fluoropolymer gasketing elements having improved "creep" or "cold flow" characteristics.
2. Description of Related Art
Polytetrafluoroethylene (PTFE) has demonstrated utility as a material in many different areas. As an industrial material, such as for example a gasket, PTFE has exhibited utility as a material for use in harsh chemical environments, which normally degrade many conventional metals and polymeric materials. PTFE is also usable over a broad temperature range, from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous polytetrafluoroethylene gasket materials, which have been compression molded or extruded and subsequently heated to a temperature above 345.degree. C., exhibit poor mechanical properties, such as low tensile strength and low "cold flow" resistance. Cold flow (also known as "creep") is a particular problem with PTFE materials, which have a tendency to spread out under compressive load, thereby reducing their effective sealability over time. Accordingly, despite a number of highly desirable properties, use of these materials are generally limited to situations not requiring high tensile strength or good creep resistance.
Polytetrafluoroethylene may be produced in a porous expanded form as taught in U.S. Pat. No. 3,953,566 to Gore. Porous expanded polytetrafluoroethylene (ePTFE) is of a higher strength than unexpanded forms of PTFE, has the chemical inertness of conventional PTFE, and, when used as a gasket, has even a wider temperature range of use up to as high as 315.degree. C. An example of a porous expanded polytetrafluoroethylene gasket material is GORE-TEX.RTM. Joint Sealant (manufactured by W. L. Gore & Associates, Inc., Elkton, Md.). Porous ePTFE joint sealants provide an excellent seal in applications having relatively wide sealing surfaces and where sufficient clamping loads can be applied to them so that, as the sealant spreads between the sealing surfaces to form a thin, wide gasket, the level of compressive stress needed to densify the gasket and to provide the desired sealability is developed. Consequently, these gaskets are often not well suited to applications having narrow sealing surfaces or requiring relatively thick gaskets.
Porous ePTFE gasket materials that have not been fully compressed will not perform as a barrier to organic liquids having surface tensions under 50 dynes/cm.sup.2. Also, they will exhibit a higher measure of creep, which results in a loss of thickness and an increased width of the gasket over time. Both the loss of gasket thickness and the increase in the gasket width contribute to a lower surface stress of the gasket in service.
Polytetrafluoroethylene sealant materials consisting of polytetrafluoroethylene cores upon which are wrapped tapes of polytetrafluoroethylene are known in the art. For example, GORE-TEX.RTM. Valve Stem Packing (manufactured by W. L. Gore & Associates, Inc., Elkton, Md.) and INERTEX Valve Stem Packing (available from Inertech, Inc., Monterey Park, Calif.), consist of porous expanded polytetrafluoroethylene cores upon which are wrapped tapes of porous expanded polytetrafluoroethylene. This type of commercially available wrapped PTFE sealant material is suitable for use as a compression packing where it is confined within a defined volume; however, when used as a gasket in an unconfined volume, these gaskets exhibit undesirable creep characteristics over a period of time when under a compressive load.
U.S. Pat. No. 5,160,773 to Sassa describes a sealing material of polytetrafluoroethylene felt encapsulated by a porous polytetrafluoroethylene sheet laminated to a melt-processible thermoplastic fluoropolymer. The encapsulated PTFE felt material is engineered for use as a "wiper" seal for moving surfaces in which very low clamping forces are applied to the seal surfaces to obtain a seal against low pressure fluids. The encapsulated felt material is generally not suited for use as a static seal against gases or fluids under pressure as a very high clamping load is required to compress it and, as with the materials described above, the encapsulated felt material deforms to a thin, wide shape as it becomes fully compressed and can exhibit undesirable creep characteristics. While the outer wrap of the encapsulated felt serves as a liquid-impermeable seal, it does not have sufficient strength to retard cold-flow of the core under high compressive stresses.
As a result of dimensional changes due to cold flow or creep, many commercially available PTFE gaskets require additional clamping force applied to the gasket some period of time after installation of the gasket. This is a highly undesirable property in a gasket material. Repetitive adjustment of clamping force is not possible in uses where a specific height of gasket must be maintained during use for functional or safety reasons. For example, when used as static seals between multiple plates of plate-and-frame heat exchangers and plate-and-frame filters any creep-induced thickness reduction of the gasket material between the plates is additive and, when multiplied by the number of plates in the unit, can result in a significant reduction in the gasket clamping load, which can lead to severe leakage problems that cannot be corrected.
Although it has not been a problem to produce a "form in place" conformable PTFE gasket material that has strength oriented in the longitudinal direction of the material, this material tends to be limited in its functional properties. This is due to the manner in which the "form in place" material is expanded in this one direction. This processing creates orientation, and therefore strength, primarily in just the longitudinal direction. Thus the resistance to cold flow is primarily in just the longitudinal direction. However, for optimum sealing performance in a gasket, resistance to cold flow is desired in both the longitudinal and transverse directions.
W. L. Gore & Associates, Inc., produces a number of gasket materials that comprises a porous expanded PTFE core wrapped in a tape of expanded PTFE material. Various forms of this material are the subject of a number of co-pending United States patent applications. One commercially available example of this material is sold as GORE-TEX.RTM. gasket material for use in sealing plate-and-frame heat exchangers. The advantage of this two-stage construction is that the wrapping of the core material with a high strength tape ends to resist the flow of the core material out of its initial dimensions (i.e., the tape wrap confines the core from spreading out transversely under stress). While this material works quite well, it is believed that even further improvements may be possible in ease in processibility as well as creep characteristics and other properties of the gasket material.
Accordingly, it is a primary purpose of the present invention to provide a gasket that has the beneficial properties of expanded PTFE while being resistant to creep.
It is a further purpose of the present invention to provide a PTFE gasket that has creep resistance in both a longitudinal and transverse direction.
These and other purposes of the present invention will become evident from review of the following specification.